Metallic vapor discharge lamp



March 26, 1940. P, FREEDMAN 2,194,652

METALLIC VAPOR DISCHARGE LAMP Filed NOV. 29, 1937 Fig. 1

/A/VENTR Patented Mar. 26, 1940 METALLIC VAPOR DISCHARGE LAMP Paul Freedman, London, England, assis-nor to Crompton' Parkinson Limited, Guiseley, near Leeds, England Application November 29, i937, Serial No. 176,962

In Great Britain December l, 1936 y s claims.

The present invention relates to improvements in and relating to metallic vapor discharge lamps of the super high pressure mercury vapor lamp type, i. e., of the type operating at pressures above 4 atmospheres, e. g., from 5 atmospheres up to 100 atmospheres or more.

, It has long been known that mercury vapor lamps with vaporizable electrodes in very narrow silica tubes with relatively thick walls and artificially cooled, for example by a stream of water in contact with the tube exterior, could be operated at a very high vapor pressure .of several atmospheres or more, and that lamps operated at such Yvery high vapor pressures were much more efiicient and emitted a light of a color much closer to sunlight than mercury vapor lamps in which the vapor pressure did not exceed an atmosphere.

It was found, however, that the voltage drop across such super high pressure mercury vapor lamps was very high. To overcome this objectionable feature it has been proposed to employ, in the place of vaporisable electrodes, solid incandescent electrodes, preferably of the oxidecoated type. It was found that such incandescent electrodes had a short life unless they were almost completely immersed in a quantity of vaporizable metal which served to lower their temperature. Even under the most favorable conditions, however; it was .found that the voltage drop across the super high pressure mercury Avapor lamp remained high, exceeding 150 volts per centimetre of the discharge path under normal operating conditions and at higher vapor pressures corresponding to a higher luminous efhciency attaining still higher values, even as high as 300 volts/cm.

The object of the invention is to provide means whereby thevoltage drop in super high pressure 40 mercury vapor lamps is materially reduced.

According to the present invention I provide a super high pressure mercury vapor lamp in which during operation nely divided electron emissive material is distributed in the region of the discharge.

Distribution of the aforesaid electron' emissive compound may be attained by first incorporating it in a heat vaporizable electrode, such as an electrode of mercury, or mercury amalgamated with another metal, with the result that as said electrode vaporises the electron emissive material is freed for distribution in the region of the discharge and around the inner wall of the envelope or alternatively such electron emissive material may be introduced as a non-adherent coating on the surface of an electrode of the solid incandescent kind, the electron emissive material being again distributed by the discharge. In the latter case the electrode may advantageously have an adhesive under coating of an electron emissive g material;

One or both of the electrodes of a super high pressure mercury vapor lamp may be formed in a manner according to the present invention, it being imperative of course that the electrode or 10 electrodes serving as a cathode or cathodes should be so treated and it is foundthat in some cases the besty results are achieved by incorporating in the lamp at least one heat vaporzable electrode.

The envelope of a lamp according to the presl5 ent invention preferably'comprises a silica envelop with a narrow bore and thick walls, such envelope containing a permanent filling of a rare gas such as argon, neon or a mixture of rare gases at a low pressure.

The aforesaid electron emissive material must of course be chemically inert with respect to the envelope of the lamp and Vmay advantageously consist of thorium om'de.

Upon application of appropriate potential to 25 the terminals of a discharge lamp constructed according to the invention, an electric discharge passes through the permanent gas filling. This discharge causes some of the mercury to evaporate and at the same time heats some of the 30 particles of the electron emissive powder. The particles of the electron emissive powder so heated act as sources of electron emission and, in the rst place, cause an increase ofv electron emission at the surface of the electrode functioning as a 35 cathode. In an alternating current lamp each electrode functions alternatively as an anode and as a cathode and the increase of electron emission due to the heated particles of the electron emissive powder is manifested at both elec- 40 trodes. With further increase of discharge current more mercury is evaporated and, at the same time, more particles of the powder at the electrodes, in the region of the discharge, and on the inner wall of the envelope, are heated 45 to a temperature at which they are capable of high electron emission. In this manner the conductivity of the entire discharge region is increased and the voltage drop across the lamp is reduced in a satisfactory manner. The effect 50 is further enhanced by secondary electron emission due to collisions of particles of the electron emissive powder with primary electrons.

In order that my invention may be well understood I will now describe by way of example one embodiment thereof with reference to the accom' electron emissive powder and consisting for exam ple of a tungsten wire coated with nely divided refractory electron emissive material such as thorium oxide.

The rear end of each of the electrodes 3, 3 is secured to a seal 5, 5 consisting of a -short ribbon of molybdenum fused to the envelope I and a lead 6, 6, preferably consisting of twisted molybdenum Wire extends from each sealing ribbon 5, E, through the ends of the envelope i and serves for the external connection to any desired leads.

Whilst I have hereinbefore described one embodiment of my invention I wish it to be understood that the specific details may be varied or modified withoutdeparting from the scope thereof and therefore my invention is not to be limited by the description but solely by the appended claims.

I claim: x

1. A super high pressure mercury vapor discharge lamp comprising an envelope, electrodes sealed into said envelope and electron emissive material disposed within said envelope and applied to a carrier in a loosely adherent manner so that during normal operation of said lamp said electron emissive material readily becomes distributed in the region of the discharge space of said lamp.

2. A super high pressure mercury vapor disf charge lamp comprising an envelope, electrodes, atleast one of which is a heat vaporizable liquid electrode, and finely divided electron emissive `material incorporated in said heat vaporizable liquid electrode so that during normal operation of said lamp, said electron emissive material readily becomes distributed in the region of the discharge space of said lamp.v

3. A super high pressure mercury vapor discharge lamp comprising an envelope, electrodes at least one of which consists of one of the group comprising mercury and mercury amalgams, and

ilnely' divided electron emissive material incorporated in said last mentioned electrode so that during normal operation of said lamp, said electron emissive material readily becomes distributed in the region of the discharge space of said lamp.

4. A super high pressure mercury hvapor discharge lamp comprising an envelope, electrodes, at least one of which is of the solid incandescent kind, and electron emissivematerial applied to said solid electrode in a loosely adhering manner so that during normal operation of saidA lamp, said electron emissive material readily becomes distributed in the region of the discharge space of said lamp.

5. A super high pressure mercury vapor discharge lamp comprising an envelope, electrodes, at least one ofwhich is of the solid incandescent kind, electron emissive material applied to said solid electrode in a strongly adhering manner and further electron emissive material applied to said solid electrode in a loosely adhering manner so that during Anormal operation of said lamp, said last mentioned electron emissive material readily becomes distributed in the region of the discharge space of said lamp.

6. A super high pressure mercury vapor discharge lamp comprising an envelope, electrodes, of which at least one is of the solid incandescent kind, a thin Wire wound around the inner end of said solid electrode, electron emissive material applied to said thin wire in a strongly adhering manner and further electron emissive material applied to said thin wire ina loosely adhering manner so that during normal operation of said lamp, said last mentioned electron emissive material readily becomes distributed in the region of the discharge space of said lamp.

7. A super high pressure mercury vapor lamp comprising an envelope, electrodes and finely divided thorium oxide applied to at least one of ysaid electrodes in a loosely adhering manner so that during normal operation of said lamp, said thorium oxide readily becomes distributed in the region of the discharge space of said lamp.

8. A super high pressure mercury vapor discharge lamp comprising an envelope, electrodes, iinely divided electron emissive material applied to said electrodes in a loosely adherent manner so that said material readily becomes detached therefrom and distributed in the region of the discharge space of said lamp and mercury and a rare gas at low pressure located within said lamp.

PAUL FREEDMAN. 

